Abrasive recovery method and abrasive recovery device

ABSTRACT

There is provided an abrasive recovery device and an abrasive recovery method capable of recovering a slurry which is condensed until a concentration of its abrasive becomes high while suppressing an increase in pressure loss and a great decrease in a recovery ratio ascribable to membrane clogging. The abrasive recovery device is a device  1  which recovers an abrasive from a used polishing slurry which has been used in a CMP process, the device including a separation membrane  41  having a cylindrical hole passage to which the used polishing slurry is led, wherein an effective filtration part of the hole passage of the separation membrane  41  has a 0.8 m length or less, and the abrasive recovery device  1  condenses the used polishing slurry until a concentration of the abrasive becomes 10 mass % or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior International ApplicationNo. PCT/JP2011/004593 filed on Aug. 16, 2011, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2011-039948 filed on Feb. 25, 2011; the entire contents of all of whichare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a recovery method and a recovery deviceof an abrasive, and more particularly to an abrasive recovery methodcapable of condensing a used polishing slurry to a high concentrationand to a recovery device using the same.

BACKGROUND

A surface of a coating film such as an insulating film or a metal thinfilm formed on a semiconductor wafer is required to be a surface havinghigh planarity. To meet the requirement, CMP (Chemical MechanicalPolishing) which performs the polishing while a polishing slurry isinterposed between a polishing member such as a polishing pad and thesemiconductor wafer is adopted.

As an abrasive used in CMP, silica fine particles high in dispersibilityand having a uniform particle size, ceria high in polishing speed,alumina having high hardness and stability, and the like are used. Theseabrasives are provided by makers as slurries in which particles withpredetermined particle size are dispersed in water in the predeterminedconcentration. When supplied to a CMP machine, the slurry is diluted toa predetermined concentration for use according to each site.

Generally, in addition to the abrasive, a pH adjusting agent such aspotassium hydroxide, ammonia, organic acid, or amines, a dispersingagent such as a surfactant, an oxidant such as hydrogen peroxide,potassium iodate, or iron nitrate (III), and so on are added to theslurry in advance. Alternatively, these components are added separatelyto the slurry at the time of the polishing.

The reuse of these polishing slurries is desired in view of that theyare used in large amount and are expensive and from a viewpoint ofreducing an amount of industrial wastes. However, wastewater of apolishing process is diluted by water and the like in many washingprocesses and the concentration of the abrasive therein is decreased. Inaddition, in the wastewater of the polishing process, fine particlesproduced as a result of breakage of the semiconductor wafer, a materialof the coating film, chips of the polishing pad, and the abrasive, solidimpurities with a large particle size produced by the flocculation ofthe abrasive, and so on are mixed. Therefore, if such wastewater of thepolishing process is reused as the abrasive without any treatment, aspeed of the polishing for flattening is lowered due to the reduction inthe concentration of the abrasive, or yields of products lower due tothe occurrence of scratches on a surface of the wafer.

Therefore, before the reuse, it is necessary to remove impurities suchas coarse solids and salts from the polishing wastewater and furtherperform the condensation treatment to re-prepare the polishing slurrywith a predetermined composition.

Developments of various techniques have been conventionally attemptedfor the treatment of the wastewater of the CMP process. For example,there has been proposed a method in which wastewater of a CMP process issubjected to membrane treatment by being passed through a separationmembrane such as a micro-filtration membrane or an ultra-filtrationmembrane, a chemical agent and ultra-pure water are added to prepareconcentrations of an abrasive and a dispersing agent, and the resultantis reused as a polishing slurry.

A related reference, for instance, discloses a membrane treatment methodof CMP wastewater in which, in order to improve a membrane permeationrate, a circulating liquid undergoes membrane separation in a firstmembrane element while its slurry concentration is maintained at apredetermined value, and thereafter, part thereof is extracted tofurther undergo membrane separation in a second membrane element.

Another related reference discloses a separation method of a slurriedpolishing liquid for semiconductor capable of removing a flocculatewhose size becomes large due to the flocculation of fine particles andcapable of recovering abrasive particles with a desired particle size.In Another related reference, the fine particles are removed by a “1 m”ultra-filtration membrane of a first step and coarse particles causing apolishing scratch are removed by a “2 m” micro-filtration membrane of asecond step.

Further, another related reference discloses a method in which abrasivewastewater of a CMP process is condensed so that its specific gravitybecomes 0.90 to 0.96 times a specific gravity of an undiluted solutionby using a condensation filter such as an ultra-filtration membrane(primary condensation step) and this condensate is further condensed sothat its specific gravity becomes 0.99 times to 1.01 times the specificgravity of the undiluted solution (secondary condensation step), wherebyan abrasive is recycled.

One of the related references discloses an abrasive recovery devicewhich reduces an amount of water passed through a membrane separationmeans and an amount of a dispersing agent in wastewater to suppressloading of the membrane at an early stage. In the reference, acondensate having undergone the condensation by a first membraneseparation means on a pre-stage is led to a second membrane separationmeans on a post-stage and coarse solids are removed by the secondmembrane separation means.

As hollow-fiber separation membranes used in the above-describedexamples, ultra-filtration membranes are widely used. With respect tothe hollow-fiber separation membrane, it is excellent in view of cost asits effective filtration length is longer because the number of modulesused is reduced and those with an about 1 m filtration length are widelyused in general

On the other hand, in the hollow-fiber separation membrane, as water tobe treated has a higher concentration, solid components are deposited ascakes on its inner walls and a thickness of the cakes graduallyincreases. This results in a decrease in an effective inside diameter ofthe membrane to cause an increase in pressure loss and the occurrence ofclogging of the membrane, which is liable to greatly lower a recoveryratio of the abrasive. Especially in the case of the treatment ofwastewater of the CMP process, this tendency becomes conspicuous whenthe concentration of the abrasive exceeds about several %. Therefore,actually, the condensation up to about several % is a limit. In the artsdescribed in the aforesaid Patent Documents as well, when the water tobe treated with a high concentration is passed, it is difficult tosufficiently suppress the loading of the separation membrane and anaccompanying reduction in the recovery ratio of the particles of theabrasive.

Generally, in filtration by a separation membrane, a progress speed ofloading of the membrane rapidly increases when a concentration of waterto be treated exceeds a predetermined value. As previously described, inthe case of the polishing slurry, the loading rapidly progresses whenthe concentration of the abrasive exceeds about several %. Therefore, inconventional arts of wastewater treatment or the like, as theconcentration of a solid component, about several % is a limit inpractice, and with such a concentration, the reuse directly in the CMPprocess as a polishing slurry is difficult.

Further, when a hollow-fiber separation membrane is used, if the slurryis condensed until the concentration of the abrasive in the slurryexceeds about several %, the deposition of the cakes graduallyprogresses in the hollow fibers as previously described. It has beenfound out that, if the condensation is further continued thereafter, theabrasive turned into a gel state is deposited on a treatment wateroutlet side of the hollow fibers as shown in FIG. 8. In this state, acondensate cannot be obtained, and further the continuation of the useof the module itself becomes impossible. Further, since the abrasive isdeposited in the gel form, a recovery ratio of the abrasive rapidlydecreases.

The present invention was made to solve the aforesaid problems, and anobject thereof is to provide an abrasive recovery device and an abrasiverecovery method capable of suppressing an increase in pressure loss anda great reduction in a recovery ratio ascribable to the clogging of amembrane and capable of condensation so that a concentration of anabrasive becomes high.

As a result of studious studies by the present inventor in order toachieve the above object, it has been found out that whether or notcakes are deposited on filtration surfaces of a separation membrane andwhether or not a gelatinous substance of an abrasive is deposited on atreatment water outlet side as described above depend mainly on aneffective filtration length of the separation membrane, which has led tothe completion of the present invention.

An abrasive recovery device of the present invention is a device whichrecovers an abrasive from a used polishing slurry in a CMP process, theabrasive recovery device including a separation membrane having acylindrical hole passage to lead the used polishing slurry, wherein aneffective filtration part of the hole passage of the separation membranehas a 0.8 m length or less, and the abrasive recovery device condensesthe used polishing slurry until a concentration of the abrasive becomes10 mass % or more.

The concentration of the abrasive led to the separation membrane dependson a concentration of wastewater of a customer's factory and thereforeis not particularly limited, but generally, a used slurry with 0.02 to 5mass % can be led. A hollow part of the separation membrane ispreferably passed through by the used polishing slurry in a cross-flowmethod. The separation membrane is preferably provided in a membraneseparation unit of an internal-pressure type. The separation membrane ispreferably a hollow-fiber membrane. An inside diameter of the separationmembrane is preferably not less than 0.1 mm nor more than 0.8 mm. Amolecular cut-off of the separation membrane is preferably 3,000 to30,000. The separation membrane is preferably made of any one ofpolyethylene, tetrafluoroethylene, polyvinylidene difluoride,polypropylene, cellulose acetate, polyacrylonitrile, polyimide,polysulfone, and polyethersulfone.

The abrasive recovery device can have a pre-separation membrane providedon a previous stage of the separation membrane, having a longereffective filtration length than an effective filtration length of theseparation membrane, and having a cylindrical hole passage. Namely, Anabrasive recovery device of the present invention can include a firstseparation membrane having a first cylindrical hole passage to lead aused polishing slurry in a CMP process, the first cylindrical holepassage having a first effective filtration part, a second separationmembrane provided on a subsequent stage of the first separationmembrane, the second separation membrane having a second cylindricalhole passage to lead a condensate from the first separation membrane,the second cylindrical hole passage having a second effective filtrationpart shorter than the first effective filtration part in length, thesecond effective filtration part being not longer than 0.8 m,circulation mechanism configured to pass the condensate from the firstseparation membrane through the second separation membrane sequentiallyand condense the used polishing slurry until a concentration of theabrasive becomes 10 mass % or more and recover an abrasive from the usedpolishing slurry.

Preferably, a length L1 of the first effective filtration part is 0.8 to1.5 m, and a length L2 of the second effective filtration part is 0.2 to0.8 m or less.

An abrasive recovery method of the present invention includes: passing aused polishing slurry in a CMP process through a separation membranewhose hole passage is cylindrical and whose effective filtration parthas a 0.8 m length or less; and condensing the used polishing slurryuntil a concentration of an abrasive of the used polishing slurrybecomes 10 mass % or more. Preferably, a hollow portion of theseparation membrane is passed through by the used polishing slurry in across-flow method. Further, an inside diameter of the separationmembrane is preferably not less than 0.1 mm nor more than 0.8 mm.Further, a circulation flow rate of water to be treated in the effectivefiltration part of the separation membrane is preferably 0.5 to 2 m/sec.

The method may include: a first filtration step of passing the usedpolishing slurry in the CMP process through a pre-separation membranewhose hole passage is cylindrical, to condense the used polishingslurry; and a second filtration step of passing a condensate from thefirst separation membrane through a post-separation membrane whoseeffective filtration part has a 0.8 m length or less, to condense thecondensate, wherein, as the pre-separation membrane, a separationmembrane having a longer effective filtration length than an effectivefiltration length of the post-separation membrane is usable. Namely, themethod may include: a first filtration step of passing the usedpolishing slurry in the CMP process through a first separation membranehaving a first cylindrical hole passage, to condense the used polishingslurry, the first cylindrical hole passage having a first effectivefiltration part; and a second filtration step of passing a condensatefrom the first separation membrane through a second separation membraneprovided on a subsequent stage of the first separation membrane, thesecond separation membrane having a second cylindrical hole passage, thesecond cylindrical hole passage having a second effective filtrationpart shorter than the first effective filtration part in length, thesecond effective filtration part being not longer than 0.8 m, tocondense the condensate until a concentration of an abrasive of thecondensate becomes 10 mass % or more.

The concentration depends on an abrasive concentration of wastewater ofa customer's factory and therefore is not particularly limited, but inthe first filtration step, it is generally preferable that the usedpolishing slurry with 0.02 to 5 mass % is condensed to 13 mass % at themaximum, more preferably 9 to 10 mass % by the filtration, and in thesecond filtration step, it is preferable that the condensate with 13mass % or less obtained in the first filtration step is condensed up to26 mass % at the maximum, more preferably 20 to 25 mass % by thefiltration. Further, the abrasive recovery method preferably uses theabove-described abrasive recovery device of the present invention.

According to the used abrasive recovery device of the present invention,the use of the separation membrane whose effective filtration part has alength equal to a predetermined value or less makes it difficult tocause problems such as the deposition of cakes on filtration surfacesand the deposition of a gelatinous abrasive on a treatment water outletside. This makes is possible to condense the slurry until the abrasiveconcentration becomes about several ten % or more. Further, with lowerenergy than is necessary conventionally, it is possible to condense theused polishing slurry having been used in the CMP process to a highconcentration, and it is possible to recover the condensed slurry inwhich the concentration of the abrasive is increased up to a high levelenabling the reuse as a product. Further, in spite of the condensationup to such a high concentration, it is possible to suppress an increasein pressure loss inside the membrane and the clogging of the separationmembrane, so that the abrasive recovery device can be a device in whicha great decrease in a recovery ratio is suppressed.

Further, in the used polishing slurry recovery method of the presentinvention, it is possible to recover the condensed slurry in which theconcentration of the abrasive is increased up to a high level enablingthe reuse as a product without causing a great decrease in a recoveryratio. This makes it possible to cut down an amount of a new slurry usedin the CMP process by 60% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic structure of an abrasiverecovery device according to one embodiment of the present invention.

FIG. 2 is an enlarged view of a cross section of one hollow fiberforming a separation membrane 41.

FIG. 3 is a diagram showing a schematic structure of an abrasiverecovery device according to one embodiment of the present invention.

FIG. 4 is a chart showing a relation between a concentration of anabrasive in a treatment tank and an amount of a permeate discharged froma permeate outlet pipe (flux).

FIG. 5 is a chart showing a relation between a concentration of anabrasive in a treatment tank 13 and an amount of a permeate dischargedfrom a first permeate outlet pipe 22 (flux), and a relation between aconcentration of the abrasive in a treatment tank 17 and an amount of apermeate discharged from a second permeate outlet pipe 24 (flux), in therecovery device in FIG. 3.

FIG. 6 is a photograph showing a state of an inlet of a membraneseparation unit at an instant when a separation membrane is clogged.

FIG. 7 is an enlarged photograph of FIG. 6.

FIG. 8 is a photograph showing a state of an outlet of the membraneseparation unit at the instant when the separation

DETAILED DESCRIPTION

Hereinafter, an abrasive recovery device and an abrasive recovery methodof the present invention will be described in detail.

First Embodiment

FIG. 1 is a diagram showing a schematic structure of an abrasiverecovery device according to one embodiment of the present invention. Inthe abrasive recovery device 1 in this embodiment, a guard filter 2which removes coarse particles contained in a used polishing slurry Swhich has polished a semiconductor in a CMP process (hereinafter,referred to as the used polishing slurry S), a treatment tank 3 housingwater to be treated from the guard filter 2, and a membrane separationunit 4 including a separation membrane 41 which filtrates the usedpolishing slurry S are installed in sequence along a flow path.

Note that the guard filter 2 captures solid impurities with a largeparticle size produced by the flocculation of an abrasive, polishing padchips when a semiconductor wafer is polished, and so on. As the guardfilter 2, any filter is usable without any particular limitation,provided that it has a larger pore size than a particle size ofparticles of the abrasive.

The guard filter 2 and the treatment tank 3 are connected by a pipe 5.The treatment tank 3 and the membrane separation unit 4 are connected bya pipe 6 having a pump P1. Note that the treatment tank 3 is providedwith a component concentration meter C1.

A permeate outlet pipe 7 and a condensate outlet pipe 8 having anopening/closing valve B1 are connected to the membrane separation unit4. The condensate outlet pipe 8 is opened so as to supply a condensateobtained in the membrane separation unit 4 to a condensate recovery tank9.

Between an upstream portion of the opening/closing valve B1 of thecondensate outlet pipe 8 and the treatment tank 3, there is provided areflux pipe 10 through which the condensate obtained in the membraneseparation unit 4 flows back to the treatment tank 3 while theopening/closing valve B1 is closed and the opening/closing valve B2 isopened.

The separation membrane 41 has cylindrical hole passages. The usedpolishing slurry S is passed inside or outside the hole passages, sothat excessive water of the used polishing slurry S is removed,resulting in the condensation.

As the separation membrane 41 having the cylindrical hole passages, aseparation membrane of a hollow-fiber type, a tubular type, or aflat-membrane type is applicable, for instance. Among them, thehollow-fiber separation membrane is space-saving and can have a largemembrane area and thus is suitably used as the separation membrane 41.

A length L of an effective filtration part of the separation membrane 41is 0.8 m or less, preferably 0.5 m or less, and more preferably 0.3 m orless depending on a targeted condensation concentration. Generally, inthe case of the hollow-fiber separation membrane, for instance, when ahigh-concentration slurry is passed through the separation membrane,solid components are deposited on filtration surfaces 412 of hollowfibers 410 in a process where the condensate passes through itseffective filtration parts. Further continuing the water passage resultsin the formation of cake layers due to the solid components deposited onthe filtration surfaces 412 to cause an increase in their thickness(refer to FIG. 2). The longer the effective filtration length, the morelikely the cake layers are formed on the filtration surfaces 412 of theseparation membrane 41. The formation of the cake layers narrows aneffective inside diameter 410S of the hollow fibers 410, so that apressure loss increases or the membrane is clogged, which sometimesgreatly lowers recovery efficiency of the abrasive. Further, since thecake layers and the gelatinous deposits are formed of abrasiveparticles, a recovery ratio of the abrasive particles lowers inaccordance with the increase of the cake layers and the generation ofthe gelatinous deposits.

Further, as shown in FIG. 8, the abrasive is deposited in a gel form ona treatment water outlet side of the hollow fibers to make it difficultto continue the filtration. Further, an amount of chemicals required forwashing the filtration surfaces and a washing time increase, causing anincrease in cost required for the whole abrasive recovery step.

In the abrasive recovery device 1 of the present invention, a separationmembrane in which the length L of its effective filtration part is 0.8 mor less, preferably 0.5 m or less, and more preferably 0.3 m or less isused as the separation membrane 41. When a module whose effectivefiltration length is such a predetermined value or less is used, eventhe passage of the used slurry with a high concentration does not easilycause loading. Therefore, the growth of the cake layers on thefiltration surfaces 412 is suppressed and the gelatinous deposits arenot generated.

Therefore, even the passage of high-concentration water to be treateddoes not easily cause an increase in pressure loss in the separationmembrane 41 and the clogging of the separation membrane 41, and a greatreduction in the recovery ratio is suppressed in the recovery device 1.

Further, when the cake layers are formed on the filtration surfaces 412as described above, the coarse particles which are gelated abrasiveparticles exfoliate from the cake layers and sometimes mix in the waterto be treated. The mixture of such coarse particles in the recoveredabrasive causes a scratch on a wafer surface when it is reused in theCMP process, causing deterioration in yields of products.

In the abrasive recovery device 1 of the present invention, as theseparation membrane 41, a separation membrane in which the length L ofits effective filtration part falls within the aforesaid range is used.Therefore, the formation of the cake layers in the separation membraneand the generation of the coarse particles due to the deposition of thegelatinous substances are suppressed, and a mixture amount of the coarseparticles in the recovered abrasive is greatly reduced. Therefore, evenwhen it is reused in the CMP process, the abrasive capable ofhigh-precision polishing can be recovered, with almost no scratch or thelike being caused on the wafer surface.

When the length L of the effective filtration part of the separationmembrane 41 is over 0.8 m, a thickness of the cake layers is likely toincrease on the filtration surfaces 412 of the separation membrane 41,and an increase in pressure loss and the clogging of the membrane easilyoccur due to a decrease in the effective inside diameter.

The length L of the effective filtration part of the separation membrane41 preferably falls within the aforesaid range and is 0.2 m or more.When the length L of the effective filtration part of the separationmembrane 41 is less than 0.2 m, the number of modules of the separationmembrane 41 installed in the recovery device 1 increases, which makes itdifficult to install an appropriate filtration device. The length L ofthe effective filtration part of the separation membrane 41 ispreferably 0.2 to 0.3 m.

The separation membrane 41 may be an organic membrane made of an organicmaterial or may be an inorganic membrane made of inorganic ceramics.

As the organic membrane, polyethylene (PE), tetrafluoroethylene (PTFE),polyvinylidene difluoride (PVDF), polypropylene (PP), cellulose acetate(CA), polyacrylonitrile (PAN), polyimide (PI), polysulfone (PS),polyethersulfone (PES), and the like, for instance, are suitably usable.

Further, as the inorganic membrane, a ceramics material of aluminumoxide (Al₂O₃), zirconium oxide (ZrO₂), or titanium oxide (TiO₂),stainless steel (SUS), glass (SPG), or the like is usable. Among them,polysulfone (PS) and polyethersulfone (PES) are suitably usable as theseparation membrane 41.

The separation membrane 41 may be a micro-filtration membrane or anultra-filtration membrane, provided that it has a hollow-fiber shape.The ultra-filtration membrane is suitably usable as the separationmembrane 41 in view of that it recovers the particles of the abrasive inthe recovered condensate most efficiently.

A molecular cut-off of the separation membrane 41 is preferably 3,000 to30,000. When the molecular cut-off of the separation membrane 41 is lessthan 3,000, it is necessary to increase a supply pressure to theseparation membrane 41 in order to obtain a permeate bypassing the waterto be treated through the separation membrane 41. This lowers energyefficiency and may possibly impair the separation membrane 41.

On the other hand, when the molecular cut-off of the separation membrane41 is over 30,000, part of the particles of the abrasive passes throughthe separation membrane 41 to move to a permeate side, which is liableto disable the efficiently recovery of the particles of the abrasive.Further, in this case, fine particles having substantially the samediameter as a pore size of the separation membrane 41 are likely to clogthe holes of the separation membrane 41, which may cause the loading.The molecular cut-off of the separation membrane 41 is more preferably6000 to 10000.

When the separation membrane 41 is a hollow-fiber separation membrane ora tubular separation membrane, an inside diameter of each hollow fiberor the like is preferably not less than 0.1 mm nor more than 0.8 mm.When the inside diameter of each hollow fiber or the like of theseparation membrane 41 is less than 0.1 mm, a pressure loss of the waterto be treated flowing in the hollow portions of the membrane increases,which makes it difficult to obtain appropriate treatment efficiency.Further, in this case, membrane surface strength lowers, which is liableto cause breakage of the membrane in accordance with an increase of theconcentration of the water to be treated. On the other hand, when theinside diameter of each hollow fiber or the like of the separationmembrane 41 is over 0.8 mm, since a shear speed of the water to betreated flowing in the hollow portions of the membrane is low, theabrasive and other impurities are easily deposited on inner walls of thehollow fibers, which is liable to clog the hollow portions. Further, alarge amount of gel of the abrasive is produced, which is liable tolower the concentration of the abrasive in the condensate. In order toincrease the shear speed, it is necessary to make the facility large,and a large amount of energy is consumed, which is not preferable.Further, pressure resistance against an external pressure is liable tolower. The inside diameter of each hollow fiber or the like of theseparation membrane 41 is more preferably not less than 0.3 mm nor morethan 0.8 mm.

The membrane separation unit 4 may be an internal pressure-typeseparation unit which passes the water to be treated inside the hollowfibers 410 or may be an external pressure-type separation unit whichpasses the water to be treated outside support layers 411 of the hollowfibers 410. When the membrane separation unit 4 is the internalpressure-type separation unit, the solid components deposited on thefiltration surfaces 412 are exfoliated by a shear force of the water tobe treated passed in the hollow fibers 410 and the growth of the cakelayers can be suppressed, which is preferable.

With such a recovery device 1, it is possible to recover and reuse thepermeate obtained from the permeate outlet pipe 7. Concretely, forexample, it can be used as raw water of an ultra-pure water devicewithout being treated or after undergoing treatment according to qualityof the permeate. Further, it can be used as other utility than in afactory, for example, domestic water, water for cooling tower, or thelike.

Second Embodiment

FIG. 3 is a diagram showing a schematic structure of an abrasiverecovery device according to one embodiment of the present invention. Inthe abrasive recovery device 11 in this embodiment, on a subsequentstage of a guard filter 12 which removes coarse particles contained in aused polishing slurry S having polished a semiconductor in a CMP process(hereinafter, referred to as the used polishing slurry S), apre-treatment tank 13 housing treated water from the guard filter 12 anda pre-membrane separation unit 14 (hereinafter, referred to as the firstmembrane separation unit 14) including a pre-separation membrane 141(hereinafter, referred to as the first separation membrane 141) whichfiltrates the used polishing slurry S are installed in sequence along aflow path.

Note that the guard filter 12 captures solid impurities with a largeparticle size produced by the flocculation of an abrasive, polishing padchips when a semiconductor wafer is polished, and so on. As the guardfilter 12, any filter is usable without any particular limitation,provided that it has a larger pore size than a particle size ofparticles of the abrasive. The guard filter 12 and the pre-treatmenttank 13 are connected by a pipe 15. The pre-treatment tank 13 and thefirst membrane separation unit 14 are connected by a pipe 16 having apump P2. Note that the pre-treatment tank 13 is provided with acomponent concentration meter C2.

On a subsequent stage of the first membrane separation unit 14, apost-treatment tank 17 housing a condensate separated in the firstmembrane separation unit 14 (hereinafter, sometimes referred to as thefirst condensate), a post-membrane separation unit 18 (hereinafter,referred to as the second membrane separation unit 18) including apost-separation membrane 181 (hereinafter, referred to as the secondseparation membrane 181) which filtrates the first condensate suppliedfrom the post-treatment tank 17, and a recovery tank 19 which recovers acondensate (hereinafter, sometimes referred to as the second condensate)separated in the second membrane separation unit 18 are sequentiallyinstalled. Note that the post-treatment tank 17 is provided with acomponent concentration meter C3.

The first membrane separation unit 14 and the post-treatment tank 17 areconnected by a pipe 20 including an opening/closing valve B3. Thepost-treatment tank 17 and the second membrane separation unit 18 areconnected by a pipe 21 including a pump P3.

A first permeate outlet pipe 22 is connected to the first membraneseparation unit 14. Further, between a pre-stage of the opening/closingvalve B3 of the pipe 20 and the pre-treatment tank 13, there is provideda reflux pipe 23 through which the first condensate obtained in thefirst membrane separation unit 14 flows back to the pre-treatment tank13 while the opening/closing valve B3 is closed and the opening/closingvalve B4 is opened.

A second permeate outlet pipe 24 and a condensate outlet pipe 25including an opening/closing valve B5 are connected to the secondmembrane separation unit 18. The condensate outlet pipe 25 is opened soas to supply the condensate obtained in the second membrane separationunit 18 to the recovery tank 19. Between an upstream portion of theopening/closing valve B5 of the condensate outlet pipe 25 and thepost-treatment tank 17, there is provided a reflux pipe 26 through whichthe second condensate obtained in the second membrane separation unit 18flows back to the post-treatment tank 17 while the opening/closing valveB5 is closed and the opening/closing valve B6 is opened.

The first separation membrane 141 and the second separation membrane 181have cylindrical hole passages. The used polishing slurry S is passedinside or outside the hole passages, so that excessive water of the usedpolishing slurry S is removed, resulting in the condensation. As thefirst separation membrane 141 having the cylindrical hole passages, aseparation membrane of a hollow-fiber type, a tubular type, or aflat-membrane type is applicable, for instance. Among them, thehollow-fiber separation membrane is space-saving and can have a largemembrane area and thus is suitably used as the first separation membrane141 and the second separation membrane 181.

The second separation membrane 181 filtrates and further condenses thecondensate from the first separation membrane 141 provided on a previousstage to increase a concentration of its abrasive. A length L2 of aneffective filtration part of the second separation membrane 181 is 0.8 mor less, preferably 0.5 m or less, and more preferably 0.3 m or less.

The use of the separation membrane having the aforesaid effectivefiltration length as the second separation membrane 181 suppresses thegrowth of cake layers because loading of the membrane does not easilyoccur even when a high-concentration used slurry is passed therethrough.Therefore, even when the high-concentration water to be treated ispassed, an increase in pressure loss in the second separation membrane181 and the loading of the second separation membrane 181 do not easilyoccur, which can realize the recovery device 11 in which a greatdecrease in a recovery ratio is suppressed.

When the length L2 of the effective filtration part of the secondseparation membrane 181 is over 0.8 m, a thickness of the cake layers islikely to increase on filtration surfaces of the second separationmembrane 181, so that an increase in pressure loss and the clogging ofthe membrane due to the decrease in the effective inside diameter arelikely to occur. The length L2 of the effective filtration part of thesecond separation membrane 181 is especially preferably 0.2 to 0.3 m forthe same reason as that for the separation membrane 41 in the firstembodiment.

The length L2 of the effective filtration part of the second separationmembrane 181 is preferably shorter than the length L1 of the effectivefiltration part of the first separation membrane 141. That is, thelength L1 of the effective filtration part of the first separationmembrane 141 is preferably longer than the length L2 of the effectivefiltration part of the second separation membrane 181.

Consequently, in the first separation membrane 141, the used polishingslurry S low in concentration diluted by a dispersion medium isefficiently filtrated, and in the second separation membrane 181, thefirst condensate obtained in the first separation membrane 141 isfurther filtrated. Consequently, with low energy, it is possible torecover the condensed slurry in which a concentration of particles ofits abrasive is increased up to a high level enabling the reuse as aproduct.

In recent years, an amount of a used slurry discharged in a CMP processper day sometimes exceeds 1000 m³. Under such circumstances, there is ademand for a technique which removes and recovers water contained in theslurry and recovers an abrasive component by more efficient treatment.

In this embodiment, the above-described two-stage structure makes itpossible to greatly reduce a volume of the water to be treated owing tothe first separation membrane 141 with a large membrane area provided ona pre-stage, and to condense the water to be treated to a higherconcentration without causing the loading owing to the second separationmembrane 181 provided on a post stage. Therefore, it is possible toreduce the number of modules used as compared with the first embodiment.

When the length L2 of the effective filtration part of the secondseparation membrane 181 is equal to or more than the length L1 of thatof the first separation membrane 141, a thickness of the cake layers islikely to increase inside the second separation membrane 181, and anincrease in pressure loss and the clogging of the membrane are likely tooccur due to a decrease in an effective inside diameter.

The length L1 of the effective filtration part of the first separationmembrane 141 is not particularly limited, but is preferably 0.8 to 1.5 min consideration of a membrane area and the like. When the length L1 ofthe effective filtration part of the first separation membrane 141 isless than 0.8 m, the number of modules of the separation membrane 141installed in the recovery device 11 increases and an installation areaincreases, which makes it impossible to obtain a sufficient effect asthe recovery device.

On the other hand, when the length L1 of the effective filtration partof the first separation membrane 141 is over 1.5 m, there occurs a limitto an installation height of the first separation membrane 141 or theseparation membrane 141 is liable to be difficult to handle. The lengthL1 of the effective filtration part of the first separation membrane 141is more preferably 0.8 to 1.5 m.

The first separation membrane 141 may be a micro-filtration membrane oran ultra-filtration separation membrane, provided that it hascylindrical hole passages, i.e. it has a hollow-fiber shape. Suitablyusable is the ultra-filtration membrane which efficiently recovers theparticles of the abrasive (abrasive grains), keeps granularity of theabrasive particles in the recovered condensate constant, has a smallpore size, and is excellent in energy efficiency.

Further, the second separation membrane 181 may also be amicro-filtration membrane or an ultra-filtration membrane, provided thatit has a hollow-fiber shape. The ultra-filtration membrane is suitablyusable in view of keeping a recovery ratio of the abrasive particles inthe recovered condensate high.

A molecular cut-off of the first separation membrane 141 and the secondseparation membrane 181 is preferably 3,000 to 30,000. When themolecular cut-off of the first separation membrane 141 and the secondseparation membrane 181 is less than 3,000, it is necessary to increasea supply pressure to the separation membranes in order to obtainpermeates by passing the water to be treated through the separationmembranes. Accordingly, energy efficiency lowers and the separationmembranes are liable to be damaged.

On the other hand, when the molecular cut-off of the first separationmembrane 141 and the second separation membrane 181 is over 30,000, partof the abrasive particles passes through the first separation membrane141 to move to a permeate side, which may lower recovery efficiency ofthe abrasive particles. Further, in this case, holes of the separationmembrane 141 and the second separation membrane 181 are likely to beclogged by fine particles having substantially the same size as the poresize of the separation membrane 141, which may cause the loading.

When the first separation membrane 141 and the second separationmembrane 181 are hollow-fiber separation membranes or tubular separationmembranes, an inside diameter thereof is preferably not less than 0.1 mmnor more than 0.8 mm. When the inside diameter of each hollow fiber orthe like of the first separation membrane 141 is less than 0.1 mm, apressure loss of the water to be treated flowing in hollow portions ofthe membrane increases, which makes it difficult to obtain appropriatetreatment efficiency and lowers membrane surface strength of theseparation membrane 141, which may lead to a breakage of the membrane inaccordance with an increase in the concentration of the water to betreated. On the other hand, when the inside diameter of each hollowfiber or the like of the first separation membrane 141 is 0.8 mm ormore, a shear speed of the water to be treated flowing in the hollowportions of the membrane is low and the abrasive and other impuritiesare easily deposited on inner walls of the hollow portions, which mayclog the hollow portions, cause the generation of a large amount of gelproduced by the flocculation of the abrasive particles, and lower theconcentration of the abrasive in the recovered condensate. The insidediameter of each hollow fiber or the like of the first separationmembrane 141 and the second separation membrane 181 is more preferablynot less than 0.3 mm nor more than 0.8 mm.

The first separation membrane 141 and the second separation membrane 181may be organic membranes made of an organic material or may be inorganicmembranes made of inorganic ceramics. As the organic membrane,polyethylene (PE), tetrafluoroethylene (PTFE), polyvinylidene difluoride(PVDF), polypropylene (PP), cellulose acetate (CA), polyacrylonitrile(PAN), polyimide (PI), polysulfone (PS), polyethersulfone (PES), and thelike, for instance, are suitably usable.

Further, as the inorganic membrane, a ceramics material of aluminumoxide (Al₂O₃), zirconium oxide (ZrO₂), or titanium oxide (TiO₂),stainless steel (SUS), glass (SPG), or the like is usable. Among them,polysulfone (PS) and polyethersulfone (PES) are suitably usable as thefirst separation membrane 141 and the second separation membrane 181.

When the first separation membrane 141 and the second separationmembrane 181 are hollow-fiber separation membranes, separation units maybe of an internal-pressure type which passes the water to be treatedinside the hollow fibers or may be of an external-pressure type whichpasses the water to be treated outside support layers of the hollowfibers. When the separation unit is of the internal-pressure type, solidcomponents deposited on filtration surfaces are exfoliated by a shearforce of the water to be treated passed in the hollow fibers, which ispreferable because the growth of cake layers can be suppressed.

In such a recovery device 11, since it is possible to condense a dilutedsolution of the slurry to a certain degree by the first separationmembrane 141, it is possible to efficiently condense a large amount ofCMP wastewater. Especially in a semiconductor factory whose productionscale is large, an amount of wastewater discharged per day sometimesreaches 1000 tons or more, but providing the first separation membrane141 makes it possible to reduce such a large amount of the wastewater toabout 1/10 to 1/500. Therefore, as compared with the first embodimentwithout the first separation membrane 141 being provided, it is possibleto reduce the number of modules installed as a whole.

In the foregoing, the abrasive recovery device 11 of the presentinvention is described based on its example, but its structure can bechanged as necessary within the limit not contrary to the spirit of thepresent invention.

Next, an abrasive recovery method using the abrasive recovery device 11of the present invention will be described based on FIG. 3. Note thatthis embodiment will describe a case where the abrasive recovery device11 including ultra-filtration membranes of a hollow-fiber type both asthe first separation membrane 141 and the second separation membrane 181is used. The first separation membrane 141 is not necessarily thehollow-fiber separation membrane, provided that it has cylindrical holepassages, and may be of a tubular type or of a flat membrane type.

The used polishing slurry S to be treated is not particularly limited,provided that it contains an abrasive which has been used in a CMPprocess (chemical mechanical polishing process). Examples of particlesof such an abrasive are silicon particles, cerium particles, and so on.As the particles of the abrasive, those having an average particle sizeof 0.01 to 1 μm are suitably used in general. The average particle sizeof the abrasive is appropriately decided depending on the CMP processand is 0.04 to 0.4 μm, for instance.

First, the used polishing slurry S which has been used in the CMPprocess is supplied to the guard filter 12 via the pipe 15. The usedpolishing slurry S is stored in the pre-treatment tank 13 after itscoarse particles with a particle size of several ten μm or more areremoved in a process where it passes through the guard filter 12.

The concentration of the particles of the abrasive in the non-treatedused polishing slurry S is not particularly limited because it dependson a customer's factory. The abrasive concentration of the usedpolishing slurry S in the CMP process is generally 0.02 to 5 mass %.

The used polished slurry S stored in the pre-treatment tank 13 ispressure-fed to the first membrane separation unit 14 including thefirst separation membrane 141 via the pipe 16 by the pump P2 while theopening/closing valve B3 is closed and the opening/closing valve B4 isopened. The used polishing slurry S is passed inside each hollow fiberof the first separation membrane 141 by the cross-flow method after itscoarse particles with a particle size of several ten μm or more areremoved in the process where it passes through the guard filter 12, andis condensed with its excessive water being permeated in a process whereit passes through the effective filtration part of the first separationmembrane 141 (first filtration step). At this time, a dispersion mediumand so on of the used polishing slurry S flow out to the permeate outletpipe 22 via the separation membrane 141, and the particles of theabrasive in the used polishing slurry S remain on the first condensateside being the condensate from the first separation membrane 141.

The first condensate is made to flow back to the pre-treatment tank 13via the reflux pipe 23. After this step is continued for a predeterminedtime, the opening/closing valve B3 is opened and the opening/closingvalve B4 is closed at a stage when the concentration of the abrasive inthe stored water in the pre-treatment tank 13 as measured by thecomponent concentration meter C2 becomes 13 mass % at the maximum, morepreferably 9 to 10 mass %, and part of the first condensate is suppliedto the post-treatment tank 17 via the pipe 20.

A velocity of the water to be treated (the used polishing slurry S andthe first condensate) passing through the effective filtration part ofthe first separation membrane 141 is preferably 0.5 to 2 m/sec. When thevelocity of the water to be treated (the used polishing slurry S and thefirst condensate) in the effective filtration part of the firstseparation membrane 141 is less than 0.5 m/sec, the abrasive particleseasily adhere onto the filtration surfaces of the separation membrane141 to sometimes cause a decrease in an amount of the permeate. In thiscase, it is necessary to increase the number of the separation membranes141 installed, resulting in an increase in manufacturing cost of therecovery device 11. On the other hand, when the velocity of the water tobe treated in the effective filtration part of the first separationmembrane 141 is over 2.0 m/sec, an amount of liquid in contact with themembrane surface becomes excessive to sometimes cause heat generation.In this case, the separation membrane 141 and the condensate are bothliable to be damaged by the heat to deteriorate. Further, in order toimprove the velocity of the water to be treated in the separationmembrane 141, it is necessary to increase the sizes of the pipes, thevalves, and so on, resulting in an increase in manufacturing cost of therecovery device 11. The velocity of the water to be treated passingthrough the effective filtration part of the first separation membrane141 is more preferably 0.55 to 1.5 m/sec.

The first condensate stored in the post-treatment tank 17 ispressure-fed to the second membrane separation unit 18 including thesecond separation membrane 181 via the pipe 21 by the pump P3 while theopening/closing valve B5 is closed and the opening/closing valve B6 isopened. The second separation membrane 181 has the effective filtrationlength L2 which is shorter than the effective filtration length L1 ofthe first separation membrane 141 and is 0.8 m or less, preferably 0.5 mor less, and more preferably 0.3 m or less. In the second separationmembrane 181, the first condensate is passed to the hollow portion ofeach of its hollow fibers by the cross-flow method. The first condensateis condensed with its excessive water being permeated in a process whereit passes through the effective filtration part of the hollow fibers(second filtration step). At this time, a dispersion medium and so on ofthe first condensate flow out to the permeate outlet pipe 24 via theseparation membrane 181, and the abrasive particles contained in thefirst condensate remain in a second condensate side being a condensatefrom the second separation membrane 181.

A velocity of the water to be treated (first condensate) passing throughthe effective filtration part of the second separation membrane 181 ispreferably 0.5 to 2 m/sec. When the velocity of the water to be treatedin the effective filtration part of the second separation membrane 181is less than 0.5 m/sec, the abrasive particles easily adhere onto thefiltration surfaces of the separation membrane 181 to easily cause theclogging of the membrane. On the other hand, when the velocity of thewater to be treated in the effective filtration part of the secondseparation membrane 181 is over 2 m/sec, an excessive amount of energyis given to the abrasive particles, and the particles flocculate tosometimes form coarse particles. When the coarse particles mix in therecovered abrasive, a scratch occurs on the surface of the wafer or thelike when the abrasive is recycled in the CMP process, which sometimeslowers yields of products. Further, the generation of such coarseparticles causes the formation of cake layers on the filtration surfacesof the separation membrane, which may increase a washing time and anamount of chemicals used. The velocity of the water to be treatedpassing through the effective filtration part of the second separationmembrane 181 is more preferably 0.6 to 1 m/sec.

After the above-described treatment step is continued for apredetermined time, the opening/closing valve B5 is opened and theopening/closing valve B6 is closed at a stage when a concentration ofthe abrasive of the stored water in the post-treatment tank 17 asmeasured by the component concentration meter C3 reaches a targetconcentration, and part thereof is supplied to the recovery tank 19 viathe pipe 25. The concentration of the stored water in the post-treatmenttank 17 when it is supplied to the recovery tank 19 via the pipe 25 ispreferably 10 mass % or more and is 26 mass % at the maximum, and morepreferably 20 to 25 mass %.

A pressure loss of the water to be treated on the filtration surfaces ofthe second separation membrane 181 is preferably 0.1 MPa or less andmore preferably 0.08 MPa or less.

In the abrasive recovery method of the present invention, since thesecond separation membrane 181 having a shorter effective filtrationlength than the first effective filtration length is used, it ispossible to efficiently recover the condensed used polishing slurry S inwhich the concentration of the abrasive particles is increased up to ahigh level enabling the reuse as a product while suppressing an increasein pressure loss on the filtration surfaces of the second separationmembrane 181 and the clogging of the membrane.

This embodiment shows the filtration method of the internal-pressuretype which passes the water to be treated through the hollow portions ofthe hollow fibers both in the first filtration step and the secondfiltration step, but the present invention is not necessarily limited tosuch a form. For example, the filtration may be of an external-pressuretype which passes the water to be treated outside the hollow fibers.

The permeates obtained from the first permeate outlet pipe 22 and thesecond permeate outlet pipe 24 can be recovered to be reused. Forexample, they are usable as raw water or the like of an ultra-pure waterdevice without being treated or after being treated according to waterquality of the permeates. Further, they are also usable as other utilitythan in a factory, for example, domestic water, cooling tower water, andthe like.

EXAMPLES

Hereinafter, the present invention will be described in more detailbased on examples and comparative examples.

Example 1

A used slurry in a CMP process was filtrated by using the abrasiverecovery device 1 shown in FIG. 1.

As the pump P1 in FIG. 1, a LEVITRO pump “LEV300” (manufactured by IwakiCo., Ltd, product name) was used, and as the membrane separation unit 4,a hollow-fiber UF membrane module “FB02-VC-FUST653” (Daicen MembraneSystems Ltd., product name) was used. As the treatment tank 3 and thecondensate recovery tank 9, a condensation tank (made of PVC) was used.As for the hollow-fiber UF membrane module “FB02-VC-FUST653”, molecularcut-off; 6000, hollow fiber inside diameter; 0.5 mm, membrane area; 0.5m², and effective filtration length; 0.26 m.

First, a used polishing slurry solution with an abrasive concentrationof about 1 mass % (pH: 9.8) was supplied to the treatment tank 3 via theguard filter 2 through the pipe 5. Next, the used polishing slurry inthe treatment tank 3 was passed through the membrane separation unit 4while the opening/closing valve B1 was closed and the opening/closingvalve B2 was opened. A circulation flow rate of the used slurry in eachpipe was 8.1 L/min and a linear velocity in the hollow-fiber membrane 41was 0.55 m/sec. Further, an impellor rotation speed of the pump P1(LEVITRO pump) and an opening/closing degree of the opening/closingvalve B2 on the subsequent stage of the membrane separation unit 4 wereadjusted so that a pressure near an inlet of a flow of the usedpolishing slurry in the hollow-fiber membrane 41 became 0.2 MPa. In thisstate, the water passage was performed, and a concentration of anabrasive in the treatment tank 3 as measured by the componentconcentration meter C1 and an amount of a permeate (flux) dischargedfrom the permeate outlet pipe were measured.

Example 2

A used polishing slurry was passed in the same manner as in Example 1except that as the membrane separation unit 4, a hollow-fiber UFmembrane module “M81S60001N” (manufactured by SPECTRUM LaboratoriesInc., product name) was used, and a concentration of an abrasive in thetreatment tank 3 was measured in the same manner as in Example 1. As forthe hollow-fiber UF membrane module “M81S60001N”, hollow-fiber insidediameter; 0.5 mm and effective filtration length; 0.46 m.

Example 3

A used polishing slurry was passed in the same manner as in Example 1except that as the membrane separation unit 4, a hollow-fiber UFmembrane module “KM1S60001N” (manufactured by SPECTRUM LaboratoriesInc., product name) was used, and a concentration of an abrasive in thetreatment tank 3 was measured in the same manner as in Example 1. As forthe hollow-fiber UF membrane module “KM1S60001N”, hollow-fiber insidediameter; 0.5 mm and effective filtration length; 0.63 m.

Comparative Example 1

A used polishing slurry was passed in the same manner as in Example 1except that as the membrane separation unit 4, a hollow-fiber UFmembrane module “KM1S30001N” (manufactured by SPECTRUM LaboratoriesInc., product name) was used, and a concentration of an abrasive in thetreatment tank 3 was measured in the same manner as in Example 1. As forthe hollow-fiber UF membrane module “KM1S30001N”, hollow-fiber insidediameter; 0.5 mm and effective filtration length; 0.81 m.

Comparative Example 2

A used polishing slurry was passed in the same manner as in Example 1except that as the membrane separation unit 4, a hollow-fiber UFmembrane module “AMK-VC-FUST653” (manufactured by Daicen MembraneSystems Ltd., product name) was used, and a concentration of an abrasivein the treatment tank 3 was measured in the same manner as in Example 1.As for the hollow-fiber UF membrane module “AMK-VC-FUST653”,hollow-fiber inside diameter; 0.5 mm, effective filtration length; 1.0m, and membrane area; 1.5 m².

A relation between the concentration of the abrasive in the treatmenttank 3 as measured by the component concentration meter C1 and theamount of the permeate (flux) discharged from the permeate outlet pipe 7in Example 1 and the Comparative Example 2 is shown in FIG. 4. Note thatin FIG. 4, the broken line represents the amount of the permeate fromthe permeate outlet pipe 7 in Example 1 and the solid line representsthe amount of the permeate from the permeate outlet pipe 7 inComparative Example 2. Further, Table 1 shows the hollow-fiber insidediameter, the membrane area, the effective filtration length, and thematerial of the hollow-fiber separation membrane 41, and also theabrasive concentration of the water to be treated at the start of thewater passage and the abrasive concentration of the water to be treatedat an instant when the condensation became impossible (maximumcondensable concentration), in Examples 1 to 3 and Comparative Examples1 to 2.

TABLE 1 Abrasive Abrasive concen- concen- tration tration of water to ofwater to Hollow- be treated be treated fiber Mem- Effective (at thestart (at the start inside brane filtration of water of water diameterarea length Mate- passage) passage) [mm] [m²] [m] rial [mass %] [mass %]E1 0.5 0.50 0.26 PES ¹⁾ 1 25 E2 0.5 6.6 0.46 PS ²⁾ 1 22 E3 0.5 8.2 0.63PS 1 20 CE1 0.5 11.2 0.81 PS 1 17 CE2 0.5 1.5 1.0 PES 1 13 ¹⁾Polyethersulfone ²⁾ Polysulfone E1, E2, E3 = Example 1, Example 2,Example 3 CE1, CE2 = Comparative Example 1, Comparative Example 2

As is apparent from Table 1, in the recovery devices of Examples 1 to 3in which the effective filtration length of the separation membrane 41is 0.8 m or less, the treated water having the abrasive concentrationwhich is as high as 20 mass % or more is obtained at the instant whenthe filtration is ended, and it has been confirmed that the maximumcondensable concentration becomes larger as the effective filtrationlength becomes shorter. On the other hand, in the recovery device ofComparative Example 1 having the effective filtration length over 0.8 m,the maximum condensable concentration is less than 20 mass %, and it hasbeen confirmed that the loading easily occurs in a high-concentrationregion.

Further, as shown in FIG. 4, in the recovery device of ComparativeExample 2 using, as the membrane separation unit 4, the separationmembrane 41 having the effective filtration length over 0.8 m, thepermeate amount (flux) rapidly reduces at an instant when a Siconcentration of the water to be treated exceeds 13 mass %, and thefurther filtration was difficult. On the other hand, in the recoverydevice of Example 1 using, as the membrane separation unit 4, theseparation membrane 41 having the effective filtration length equal to0.8 m or less, the rapid decrease of the permeate amount did not occureven when the concentration of the water to be treated exceeds 20 mass%, and the stable filtration was possible even when the water to betreated came to have a high concentration.

From the above result, it has been confirmed that, when the separationmembrane 41 whose effective filtration length is 0.8 m or less is usedas the membrane separation unit 4, it is possible to stably condense thelow-concentration used slurry discharged from the CMP process up to ahigh-concentration range.

Conventionally, due to the need for complying with the emissionstandard, solid-liquid separation has been performed for wastewatercontaining abrasive particles such as silica particles. However, evenwhen an ultra-filtration membrane is used for the solid-liquidseparation, about several % was a limit for the condensation as a solidcomponent. A polishing slurry, if its abrasive component has aboutseveral % concentration, is difficult to reuse in the CMP process, andtherefore, the solid component has been usually disposed of asindustrial wastes.

In the present invention, as shown in Examples 1 to 3, the wastewaterfrom the CMP process can be condensed so that the concentration of itsabrasive becomes about 25% enabling the use as a product, which makes itpossible to use the recovered condensate again in the CMP process,enabling high recycling efficiency.

Next, Example 4 to 5 and Comparative Example 3 to 6 were carried out byusing the abrasive recovery device 1 used in Example 1, with thepressure at an inlet of the water to be treated in the membraneseparation unit 4 being the same as that of Example 1, and with thepressure loss of the separation membrane being varied.

Example 4

A used polishing slurry was passed through the recovery device 1 forfiltration, with a linear velocity in the hollow-fiber membrane 41 being0.6 m/sec, and with other conditions being the same as in Example 1.After this treatment was continued for 80 minutes, a condensate from themembrane separation unit 4 was recovered from the condensate outlet pipe8 to the condensate recovery tank 9 while the opening/closing valve B1was opened and the opening/closing valve B2 was closed.

Example 5

The filtration was performed in the same manner as in Example 2 exceptthat the following points were changed, and a condensate from themembrane separation unit 4 was recovered. As the membrane separationunit 4, a hollow-fiber UF membrane module “SLP-1053” (manufactured byAsahi Kasei Chemicals Corporation, product name, hollow-fiber insidediameter 1.4 mm, molecular cut-off: 10000, membrane area: 0.12 m²,effective filtration length 0.20 m, membrane material: polysulfone) wasused instead of the hollow-fiber UF membrane module “FB02-VC-FUST653”.

As for a used polishing slurry supplied to the treatment tank 3, anabrasive concentration was set to about 0.8 mass % (pH: 10.5), acirculation flow rate of the used slurry in each pipe was set to 9.0L/min, a pressure near the inlet of the flow of the used polishingslurry in the hollow-fiber membrane 41 was set to 0.2 MPa, and a linearvelocity in the hollow-fiber membrane 41 was set to 0.69 m/sec.

Comparative Example 3

The filtration was performed in the same manner as in Example 2 exceptthat the following points were changed, and a condensate from themembrane separation unit 4 was recovered. As the membrane separationunit 4, a hollow-fiber UF membrane module “AMK-VC-FUS0181” (manufacturedby Dicen Membrane Systems Inc., product name, hollow-fiber insidediameter 0.8 mm, molecular cut-off: 10000, membrane area: 0.5 m²,effective filtration length 1 m, membrane material: PES) was usedinstead of the hollow-fiber UF membrane module “FB02-VC-FUST653”, and alinear velocity in the hollow-fiber membrane 41 was set to 0.55 m/sec.

Comparative Example 4

The filtration was performed in the same manner as in Example 2 exceptthat the following points were changed, and a condensate from themembrane separation unit 4 was recovered. As the membrane separationunit 4, a hollow-fiber UF membrane module “AMK-VC-FUS03C1” (manufacturedby Dicen Membrane Systems Inc., product name, hollow-fiber insidediameter 1.2 mm, molecular cut-off: 30000, membrane area: 0.3 m²,effective filtration length 1 m, membrane material: PES) was usedinstead of the hollow-fiber UF membrane module “FB02-VC-FUST653”, and alinear velocity in the hollow-fiber membrane 41 was set to 0.85 m/sec.

Comparative Example 5

The filtration was performed in the same manner as in Example 2 exceptthat the following points were changed, and a condensate from themembrane separation unit 4 was recovered. As the membrane separationunit 4, “AMK-VC-FUS03C1” (manufactured by Dicen Membrane Systems Inc.,product name, hollow-fiber inside diameter 1.2 mm, molecular cut-off:30000, membrane area: 0.3 m², effective filtration length 1 m, membranematerial: PES) was used instead of the hollow-fiber UF membrane module“FB02-VC-FUST653”, and a linear velocity in the hollow-fiber membrane 41was set to 1.8 m/sec.

Comparative Example 6

The filtration was performed in the same manner as in Example 2 exceptthat the following points were changed, and a condensate from themembrane separation unit 4 was recovered. As the membrane separationunit 4, “AMK-VC-FUST653” (manufactured by Dicen Membrane Systems Inc.,product name. hollow-fiber inside diameter 0.5 mm, molecular cut-off:6000, membrane area: 1.5 m², effective filtration length 1 m, membranematerial: PES) was used instead of the hollow-fiber UF membrane module“FB02-VC-FUST653”. A linear velocity in the hollow-fiber membrane 41 wasthe same as that in Example 2.

Regarding Examples 4 to 5 and Comparative Examples 3 to 6, Table 2 showsthe abrasive concentration of the condensate recovered into thecondensed recovery tank 9 and a recovery ratio of the abrasive. Inaddition, Table 2 shows an abrasive concentration value in thecondensate calculated from the linear velocity in the hollow-fibermembrane 41, the hollow-fiber inside diameter, and a discharge amount ofa permeate, regarding Examples 4 to 5 and Comparative Examples 3 to 6.

TABLE 2 Linear Abrasive Abrasive velocity concentra- concentra- Hollow-in tion of tion of Effective fiber hollow- condensate condensatefiltration inside fiber (calculated (measured Recovery length diametermembrane value) value) Ratio [m] [mm] [m/sec] [mass %] [mass %] [%] E40.26 0.50 0.60 26 25 98 E5 0.2 1.4 0.69 25 22 88 CE3 1.0 0.80 0.55 17 1376 CE4 1.0 1.2 0.85 22 13 61 CE5 1.0 1.2 1.8 22 13 60 CE6 1.0 0.50 0.6017 13 79 E4, E5 = Example 4, Example 5 CE3, CE4, CES, CE6 = ComparativeExample 3, Comparative Example 4, Comparative Example 5, ComparativeExample 6

As is apparent from Table 2, in Example 4 using the separation membranewhose effective filtration length is 0.8 m or less and whosehollow-fiber inside diameter is not less than 0.1 mm nor more than 0.8mm, the abrasive concentration of the recovered condensate is over 25mass % and it has been confirmed that a higher recovery ratio can beobtained. Further, in Example 5 in which the effective filtration lengthis made shorter and a pressure loss is decreased by increasing thehollow-fiber inside diameter as compared with Example 4, a predeterminedamount or more of the permeate was obtained until the concentration fellin a relatively high range, but the condensation up to 25 mass % or morewas not possible. Further, the concentration of the abrasive containedin the recovered condensate is lower than the abrasive concentrationcalculated from the permeate amount (calculated value), and it has beenconfirmed that the recovery ratio of the abrasive slightly decreases inaccordance with an increase in the hollow-fiber inside diameter (fiberdiameter). This is thought to be because abrasive components (abrasiveparticles) adhered in the hollow fibers to form cake layers andaccordingly the effective inside diameter decreased.

On the other hand, in Comparative Examples 3, 6 using the separationmembrane whose effective filtration length is longer than 0.8 m andwhose hollow-fiber inside diameter fell within a range from 0.1 to 0.8mm, the recovery ratio of the abrasive was nearly 76% and theconcentration of the abrasive contained in the recovered condensate waslower than the abrasive concentration calculated from the permeateamount (theoretical value). Further, in Comparative Examples 4, 5 inwhich the inside diameter of the hollow fiber was increased to 1.2 mm,the recovery ratio of the abrasive was further lower. This is thought tobe because abrasive components (abrasive particles) adhered in thehollow fibers to form cake layers and accordingly an effective insidediameter decreased.

Comparative Example 7

An ultra-filtration membrane “MLE-7101H” (manufactured by Kuraray Co.,Ltd., product name. effective filtration length; 1 m, hollow-fiberinside diameter; 1.2 mm, molecular cut-off; 13000, membrane material;polysulfone) was installed as the membrane separation unit 4 in therecovery device 1 shown in FIG. 1 and the filtration was performed. Thefiltration was performed by an internal-pressure cross-flow method. Aseparation membrane was gradually clogged from a stage when aconcentration of an abrasive contained in a condensate became 14 mass %.Thereafter, the filtration was continued with a pressure to water to betreated being increased by heightening an output of the pump, but thecondensation became impossible at a stage when a Si concentration of thewater to be treated became 19 mass %. FIG. 6 shows a state of an inletof the membrane separation unit 4 at the instant when the filtrationbecame impossible. FIG. 7 shows a part of FIG. 6 being enlarged. FIG. 8shows a state of an outlet of the membrane separation unit 4 at the sameinstant in the FIG. 7.

As shown in FIG. 8, on the membrane separation unit at the instant whenthe condensation became difficult, gelatinous coarse particles being theflocculation of the abrasive were formed in a belt form on a subsequentstage in a water passage direction of the water to be treated (i.e.outlet of the membrane separation unit 4). Consequently, the hollowportions of the separation membrane 41 were clogged, and thecontinuation of the condensation became difficult.

Example 6

A used slurry in a CMP process was filtrated by using the abrasiverecovery device 11 shown in FIG. 3.

A LEVITRO pump “LEV300” (manufactured by Iwaki Co., Ltd, product name)was used as the pumps P2 and P3 in FIG. 3, a hollow-fiber UF membranemodule “AMK-VC-FUST653” (manufactured by Daicen Membrane Systems Ltd.,product name) was used as the first membrane separation unit 14, and ahollow-fiber UF membrane module “FB02-VC-FUST653” (manufactured byDaicen Membrane Systems Ltd., product name) was used as the secondmembrane separation unit 18. Further, treatment tanks (made of PE) wereused as the treatment tank 13 and the post-treatment tank 17, and acondensation tank (made of PVC) was used as the condensate recovery tank19.

As for the hollow-fiber UF membrane module “AMK-VC-FUST653” being thefirst membrane separation unit 14, molecular cut-off; 6000, hollow-fiberinside diameter; 0.5 mm, membrane area; 1.5 m², and effective filtrationlength; 1 m, and as for the hollow-fiber UF membrane module“FB02-VC-FUST653” being the second membrane separation unit 18,molecular cut-off; 6000, hollow-fiber inside diameter; 0.5 mm, membranearea; 0.5 m², and effective filtration length; 0.26 m.

First, a 200 L used polishing slurry solution with an about 1 mass %abrasive concentration (pH: 9.8) was supplied through the pipe 15 viathe guard filter 12 to the treatment tank 13. Next, while theopening/closing valve B3 was closed and the opening/closing valve B4 wasopened, a used polishing slurry in the treatment tank 13 was passedthrough the first membrane separation unit 14. A circulation flow rateof the used slurry in the pipes was 8.0 L/min, and a linear velocity inthe hollow-fiber membrane 141 was 0.7 m/sec. Further, an impellorrotation speed of the pump P2 (LEVITRO pump) and an opening/closingdegree of the opening/closing valve B4 on the subsequent stage of themembrane separation unit 14 were adjusted so that a pressure near aninlet of a flow of the used polishing slurry in the hollow-fibermembrane 141 became 0.2 MPa.

In this state, water passage was performed, and an amount of a permeate(flux) discharged from the permeate outlet pipe 22 was measured until aconcentration of an abrasive in the treatment tank 13 as measured by thecomponent concentration meter C2 became 9 mass %. An amount of the usedpolishing slurry in the treatment tank 13 at this time was about 22 L.

Next, while the opening/closing valve B4 was closed and theopening/closing valve B3 was opened, all the amount of the usedpolishing slurry in the treatment tank 13 was supplied to thepost-treatment tank 17. Next, while the opening/closing valve B5 wasclosed and the opening/closing valve B6 was opened, the used polishingslurry in the post-treatment tank 17 was passed through the secondmembrane separation unit 18. A circulation flow rate of the used slurryin the pipes was 8.0 L/m, and a linear velocity in the second separationmembrane 181 (the hollow-fiber membrane 181) was 0.7 m/sec. Further, animpellor rotation speed of the pump P3 (LEVITRO pump) and anopening/closing degree of the opening/closing valve B6 on the subsequentstage of the second membrane separation unit 18 were adjusted so that apressure near an inlet of the flow of the used polishing slurry in thesecond separation membrane 181 (the hollow-fiber membrane 181) thehollow-fiber membrane 181 became 0.2 MPa. In this state, water passagewas performed, and an amount of a permeate (flux) discharged from thepermeate outlet pipe was measured until a concentration of an abrasivein the treatment tank 17 as measured by the component concentrationmeter C3 became 25 mass %.

FIG. 5 shows a relation, in Example 6, between the concentration of theabrasive in the treatment tank 13 as measured by the componentconcentration meter C2 and the amount of the permeate (flux) dischargedfrom the first permeate outlet pipe 22, and a relation, in Example 6,between the concentration of the abrasive in the treatment tank 17 asmeasured by the component concentration meter C3 and the amount of thepermeate (flux) discharged from the second permeate outlet pipe 24.

Note that in FIG. 5, the solid line represents the permeate from thefirst membrane separation unit 14, discharged from the first permeatedischarge pipe 22 and the broken line represents the permeate from thesecond membrane separation unit 18, discharged from the second permeateoutlet pipe 24.

Based on the measurement data in Example 1 (FIG. 4) and the measurementdata of Example 6 (FIG. 5), the abrasive recovery device 1 of Example 1(first embodiment) and the abrasive recovery device 11 of Example 6(second embodiment) were designed and fabricated. It was possible tofabricate the abrasive recovery device 11 of the second embodiment, withthe number of the hollow-fiber UF membrane modules used being reduced by87% as compared with the abrasive recovery device 1 of the firstembodiment. As a result, effects of the simplification of the pipestructure, a reduction in an installation area, and cost reduction wereobtained as the whole recovery device.

What is claimed is:
 1. An abrasive recovery device, comprising; a firstseparation membrane having a first cylindrical hole passage to lead aused polishing slurry in a CMP process, the first cylindrical holepassage having a first effective filtration part, a second separationmembrane provided on a subsequent stage of the first separationmembrane, the second separation membrane having a second cylindricalhole passage to lead a condensate from the first separation membrane,the second cylindrical hole passage having a second effective filtrationpart shorter than the first effective filtration part in length, thesecond effective filtration part being not longer than 0.8 m,circulation mechanism configured to pass the condensate from the firstseparation membrane through the second separation membrane sequentiallyand condense the used polishing slurry until a concentration of theabrasive becomes 10 mass % or more and recover an abrasive from the usedpolishing slurry.
 2. The abrasive recovery device according to claim 1,wherein at least one of hollow portions of the first separation membraneand the second separation membrane is passed through by the usedpolishing slurry in a cross-flow method.
 3. The abrasive recovery deviceaccording to claim 1, wherein at least one of the first separationmembrane and the second separation membrane is provided in a membraneseparation unit of an internal-pressure type.
 4. The abrasive recoverydevice according to claim 1, wherein at least one of the firstseparation membrane and the second separation membrane is a hollow-fibermembrane.
 5. The abrasive recovery device according to claim 1, whereinat least one of inside diameters of the first separation membrane andthe second separation membrane is not less than 0.1 mm nor more than 0.8mm.
 6. The abrasive recovery device according to claim 1, wherein atleast one of molecular cut-off of the first separation membrane and thesecond separation membrane is 3,000 to 30,000.
 7. The abrasive recoverydevice according to claim 1, wherein at least one of the firstseparation membrane and the second separation membrane is made of anyone of polyethylene, tetrafluoroethylene, polyvinylidene difluoride,polypropylene, cellulose acetate, polyacrylonitrile, polyimide,polysulfone, and polyethersulfone.
 8. The abrasive recovery deviceaccording to claim 1, wherein a length L1 of the first effectivefiltration part is 0.8 to 1.5 m, and a length L2 of the second effectivefiltration part is 0.2 to 0.8 m.
 9. An abrasive recovery methodcomprising: a first filtration step of passing the used polishing slurryin the CMP process through a first separation membrane having a firstcylindrical hole passage, to condense the used polishing slurry, thefirst cylindrical hole passage having a first effective filtration part;and a second filtration step of passing a condensate from the firstseparation membrane through a second separation membrane provided on asubsequent stage of the first separation membrane, the second separationmembrane having a second cylindrical hole passage, the secondcylindrical hole passage having a second effective filtration partshorter than the first effective filtration part in length, the secondeffective filtration part being not longer than 0.8 m, to condense thecondensate until a concentration of an abrasive of the condensatebecomes 10 mass % or more.
 10. The abrasive recovery method according toclaim 9, wherein at least one of hollow portions of the first separationmembrane and the second separation membrane is passed through by theused polishing slurry in a cross-flow method.
 11. The abrasive recoverymethod according to claim 9, wherein at least one of inside diameters ofthe first separation membrane and the second separation membrane is notless than 0.1 mm nor more than 0.8 mm.
 12. The abrasive recovery methodaccording to claim 9, wherein a circulation flow rate of water to betreated in at least one of the effective filtration parts of the firstseparation membrane and the second separation membrane is 0.5 to 2m/sec.
 13. The abrasive recovery method according to claim 9, wherein,in the first filtration step, the used polishing slurry is condensed to13 mass % at the maximum by the filtration, and in the second filtrationstep, the condensate obtained in the first filtration step is condensedup to 26 mass % at the maximum by the filtration.
 14. The abrasiverecovery method according to claim 9, wherein an abrasive concentrationof the used polishing slurry led to the first separation membrane is0.02 to 5 mass %.
 15. The abrasive recovery method according to claim 9,wherein an average particle size of abrasive particles contained in theused polishing slurry is 0.01 to 1 μm.
 16. The abrasive recovery methodaccording to claim 9, wherein a circulation flow rate of water to betreated in the first effective filtration part is 0.55 to 1.5 m/sec. 17.The abrasive recovery method according to claim 9, wherein a circulationflow rate of water to be treated in the second effective filtration partis 0.6 to 1 m/sec.